1. Field of the Invention
The present invention refers to a method and an apparatus for the flux-free application of a solder to a substrate or a chip.
2. Description of Prior Art
In the prior art various methods are known by means of which solders can be applied to a substrate. The solder to be applied can be present in the form of so-called solder balls.
In one known method, solder balls are arranged on a substrate making use of a template. Subsequently, these solder balls are remelted making use of fluxes. When this method is used, the flux is applied to the substrate before the solder balls are applied so as to fix the solder balls on the substrate.
A further known method makes use of the screen printing technology. A soldering paste is here applied to the substrate by means of the screen printing technique. The soldering paste consists of fine solder balls and of a flux. When the soldering paste has been printed onto the substrate, remelting is carried out in a furnace.
The above-described screen printing method according to the prior art is disadvantageous insofar as the possibility of using the method for soldering in the so-called fine-pitch range is limited.
The use of a flux leads to increased environmental pollution when the methods described hereinbefore are used.
When aluminium is to be soldered, the oxidized surface must be cleaned prior to the soldering process, since soldering on aluminium oxide is not possible. This cleaning by means of etching is a very expensive process step.
This problem arises especially in connection with chip contacting, when the contacting is to be carried out not by means of the known bonded connections, but when a soldered connection is desired. Conventional chips are provided with so-called pads or contact pads which consist of aluminium. Although a bonded connection with these aluminium pads can be established by means of bonding wires, the production of a soldered connection requires a metallization of the initial pad with solderable metals, such as gold. Direct, soldered contacting of the chip on the aluminium pads is not possible due to the oxidation occurring, i.e. the formation of an aluminium oxide layer on the pad. The use of a flux cannot solve this problem either.